The Alichur Dome, South Pamir, Western India-Asia Collisional Zone: Detailing the Neogene Shakhdara-Alichur Syn-collisional Gneiss-Dome Complex and Connection to Lithospheric Processes

摘要

Neogene, syn-collisional extensional exhumation of Asian lower-middle crust produced the Shakhdara-Alichur gneiss-dome complex in the South Pamir. The 1 km-thick, mylonitic-brittle, top-NNE, normal-sense Alichur shear zone (ASZ) bounds the 125 x 25 km Alichur dome to the north. The Shakhdara dome is bounded by the 4 km-thick, mylonitic-brittle, top-SSE South Pamir normal-sense shear zone (SPSZ) to the south, and the dextral Gunt wrench zone to its north. The Alichur dome comprises Cretaceous granitoids/gneisses cut by early Miocene leucogranites; its hanging wall contains non/weakly metamorphosed rocks. The 22-17 Ma Alichur-dome-injection-complex leucogranites transition from foliation-parallel, centimeter- to meter-thick sheets within the ASZ into discordant intrusions that may comprise half the volume of the dome core. Secondary fluid inclusions in mylonites and mylonitization-temperature constraints suggest Alichur-dome exhumation from 10-15 km depth. Thermochronologic dates bracket footwall cooling between 410-130 degrees C from 16-4 Ma; tectonic cooling/exhumation rates (42 degrees C/Myr, 1.1 km/Myr) contrast with erosion-dominated rates in the hanging wall (2 degrees C/Myr, 0.1 km/Myr). Dome-scale boudinage, oblique divergence of the ASZ and SPSZ hanging walls, and dextral wrenching reflect minor approximately E-W material flow out of the orogen. We attribute broadly southward younging extensional exhumation across the central South Pamir between 20-4 Ma to: (i) Mostly northward, foreland-directed flow of hot crust into a cold foreland during the growth of the Pamir orocline; and (ii) Contrasting effects of basal shear related to underthrusting Indian lithosphere, enhancing extension in the underthrust South Pamir and inhibiting extension in the non-underthrust Central Pamir.